Reversible Reactions (Cambridge O Level Chemistry)

Reversible reactions

• Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when all of the reactants are used up
• In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again
• It is said that the reaction can occur in both directions: the forward reaction (which forms the products) and the reverse reaction(which forms the reactants)

 Chemical equations for reversible reactions

• When writing chemical equations for reversible reactions, two arrows are used to indicate the forward and reverse reactions
• Each one is drawn with just half an arrowhead – the top one points to the right, and the bottom one points to the left: ⇌ 

Example

• The reaction for the Haber process which is the production of ammonia from hydrogen and nitrogen:

N₂ + 3H₂ ⇌ 2NH₃

 Hydrated and anhydrous salts

• Hydrated salts are salts that contain water of crystallisation which affects their molecular shape and colour
• Water of crystallisation is the water that is stoichiometrically included in the structure of some salts during the crystallisation process
• A common example is copper(II) sulfate which crystallises forming the salt copper(II) sulfate pentahydrate, CuSO₄.5H₂O
• Water of crystallisation is indicated with a dot written in between the salt molecule and the surrounding water molecules
• Anhydrous salts are those that have lost their water of crystallisation, usually by heating, in which the salt becomes dehydrated

 Dehydration of hydrated cobalt(II) chloride:

hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water

Diagram showing the dehydration of hydrated cobalt(II) chloride 

 Hydration of cobalt(II) chloride

• When anhydrous blue cobalt(II) chloride crystals are added to water they turn pink and the reaction is reversible
• When the cobalt(II) chloride crystals are heated in a test tube, the pink crystals turn back to the blue colour again as the water of crystallisation is lost
• The form of cobalt(II) chloride in the crystals that are pink is known as hydrated cobalt(II) chloride because it contains water of crystallisation
• When hydrated cobalt(II) chloride is heated, it loses its water of crystallisation and turns into anhydrous cobalt(II) chloride:

CoCl₂.6H₂O (s) ⇌ CoCl₂ (s) + 6H₂O (l)